1. Field of the Invention
The invention relates to a pre-pit signal decoder, and more particularly to a pre-pit signal decoder having a protection unit.
2. Description of the Related Art
In a typical DVD-R/RW (Digital Versatile Disk Recordable), a pre-pit for recording the address information is additionally formed at a position between adjacent tracks in order to correctly detect the frequency of the wobbling track. FIG. 1 shows a cross-sectional view of an optical disk. Referring to FIG. 1, the number 2 represents a groove, the number 3 represents a land, and the number 4 represents a pre-pit. Because the pre-pits have been uniformly formed on an entire surface of the DVD-R/RW after the disk is manufactured, the reference addresses of the DVD-R/RW may be reproduced from the pre-pits.
According to the DVD-R/RW specification, each error correction code (ECC) block is composed of 16 sectors, and each sector is composed of 26 frames. The 26 frames may be divided into even frames and odd frames. Each frame has 8 wobble cycles, and the length of each cycle is 186T. Thus, the length of each frame is 1488T, wherein T represents a basic data cycle. Each pre-pit bit is determined by three low bits (b2, b1, b0) in each frame, as shown in Table 1.
Each ECC block contains 13 sets of pre-pit bits, so a set of pre-pit bits appears on every two frames, and the set of pre-pit bits originally appears on the position of the even frame. However, when the pre-pit bit appears on the same position for the adjacent tracks, the pre-pit bit will be shifted to the position of the odd frame. That is, if the pre-pit bit appears on the position of the even frame, it cannot appear on the position of the odd frame. On the contrary, if the pre-pit bit does not appear on the position of the even frame, it will surely appear on the position of the odd frame.
TABLE 1Informationb2b1b0Pre-pit bit is an even sync code111Pre-pit bit is an odd sync code110Pre-pit bit is 1101Pre-pit bit is 0100
A pre-pit signal is typically generated from a differential push-pull signal using a pre-pit signal detector, and then a pre-pit byte is generated using a pre-pit decoder. Since the differential push-pull signal is an analog signal, the pre-pit signal detector mostly utilizes a slicer to slice the differential push-pull signal and then generate the pre-pit signal, which may contain noise. Consequently, if the pre-pit decoder decodes using only the pre-pit bit corresponding to the pre-pit signal of Table 1, error data may be produced.